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ABSTRACT Soil microbiota plays fundamental roles in maintaining ecosystem functions and services, including biogeochemical processes and plant productivity. Despite the ubiquity of soil microorganisms from the topsoil to deeper layers, their vertical distribution and contribution to element cycling in subsoils remain poorly understood. Here, nine soil profiles (0 to 135 cm) were collected at the local scale (within 300 km) from two canonical paddy soil types (Fe-accumuli and Hapli stagnic anthrosols), representing redoximorphic and oxidative soil types, respectively. Variations with depth in edaphic characteristics and soil bacterial and diazotrophic community assemblies and their associations with element cycling were explored. The results revealed that nitrogen and iron status were the most distinguishing edaphic characteristics of the two soil types throughout the soil profile. The acidic Fe-accumuli stagnic anthrosols were characterized by lower concentrations of free iron oxides and total iron in topsoil and ammonia in deeper layers compared with the Hapli stagnic anthrosols. The bacterial and diazotrophic community assemblies were mainly shaped by soil depth, followed by soil type. Random forest analysis revealed that nitrogen and iron cycling were strongly correlated in Fe-accumuli stagnic anthrosol, whereas in Hapli soil, available sulfur was the most important variable predicting both nitrogen and iron cycling. The distinctive biogeochemical processes could be explained by the differences in enrichment of microbial taxa between the two soil types. The main discriminant clades were the iron-oxidizing denitrifier Rhodanobacter, Actinobacteria, and diazotrophic taxa (iron-reducing Geobacter, Nitrospirillum, and Burkholderia) in Fe-accumuli stagnic anthrosol and the sulfur-reducing diazotroph Desulfobacca in Hapli stagnic anthrosol. IMPORTANCE Rice paddy ecosystems support nearly half of the global population and harbor remarkably diverse microbiomes and functions in a variety of soil types. Diazotrophs provide significant bioavailable nitrogen in paddy soil, priming nitrogen transformation and other biogeochemical processes. This study provides a novel perspective on the vertical distribution of bacterial and diazotrophic communities in two hydragric anthrosols. Microbiome analysis revealed divergent biogeochemical processes in the two paddy soil types, with a dominance of nitrogen-iron cycling processes in Fe-accumuli stagnic anthrosol and sulfur-nitrogen-iron coupling in Hapli stagnic anthrosol. This study advances our understanding of the multiple significant roles played by soil microorganisms, especially diazotrophs, in biogeochemical element cycles, which have important ecological and biogeochemical ramifications.

Background and aims: Silicon (Si) deficiency is a major constraint on rice production. The objective of this study was to evaluate the long-term influence of phytolith-rich straw return and groundwater table management on labile Si fractions in paddy soil and subsequent plant Si uptake. Methods: A field experiment was conducted over 36 years in subtropical China with different application doses of phytolith-rich straw and a groundwater table of either 20 or 80 cm. An optimized sequential chemical extraction procedure allowed us to determine labile Si fractions, represented by CaCl2-Si, Acetic-Si, H2O2-Si, Oxalate-Si, and Na2CO3-Si. Additional analyses included the determination of amorphous silica particles in soil, phytoliths in supplied straw, Si in planted rice straw, and the dissolution rate of phytoliths extracted from supplied straw. Results: Long-term application of phytolith-rich straw significantly increased the H2O2-Si and Na2CO3-Si contents. The CaCl2-Si (5.21–7.91 mg kg− 1), H2O2-Si (50.0–72.4 mg kg− 1) and Na2CO3-Si (3.33–4.60 g kg− 1) contents were positively correlated with soil organic carbon. The Si content (13.6–28.9 g kg−‍1) in planted rice straw significantly (p < 0.05) increased with the application dose of phytolith-rich straw under both groundwater tables. This effect was significantly (p < 0.05) greater under 80 cm groundwater table than under 20 cm groundwater table for matching straw amendments. Conclusions: This study indicates that long-term application of phytolith-rich straw and groundwater management significantly increase soil Si bioavailability by promoting accumulation of organic matter and phytoliths, and enhancing the soil-plant Si cycle. [ABSTRACT FROM AUTHOR]